The present invention relates in general to aircraft control systems and more particularly to apparatus for measuring the true angle of attack of an aircraft from measures of longitudinal axis and normal axis accelerations in conjunction with measures of craft vertical and forward airspeed; such measure of angle of attack being derived in a manner that is independent of the aerodynamic characteristics of the aircraft and hence is applicable to a variety of airframe designs without the requirement of extensive and costly flight test calibration. Another aspect of the present invention includes the computation of precise measures of craft acceleration along the flight path and instantaneous vertical speed both of which are devoid of deleterious effects of vertical gyro erection control effects.
The angle of attack of an aircraft is a basic parameter of flight and an extremely useful one in the guidance and control of an aircraft. However, in the past, angle of attack has not been extensively used, particularly in the guidance and control of an aircraft, primarily because of the difficulty of obtaining an accurate, noise-free signal representative thereof over a wide range of aircraft operation and flight conditions. Heretofore, angle of attack has been measured by means of probes or vanes protruding from the outside surface of the aircraft and which in one way or another measured the characteristics of air flow patterns. Such devices are subject to flow anomolies due to surface shapes and other external aircraft characteristics resulting in data that had to be very highly filtered or damped in order to provide useful control information. Furthermore, such external probes are subject to inadvertent damage due to dust, ice, ground handling and the like, creating failure and maintenance problems. Supersonic speeds will, in the future, create skin heat problems rendering probes and vanes very impractical.
While some success has been obtained in measuring angle of attack without using vanes and probes, the computations required the use of vertical gyro long term pitch (and roll) attitude data. See for example applicant's co-pending U.S. application Ser. No. 699,638 filed Jan. 22, 1968, entitled "Airspeed Command Systems" now Pat. No. 3,522,729. The principal disadvantage of vertical gyro data is that it is subject to inaccuracies due to erection and drift characteristics in long term maneuvering (accelerated) flight. Furthermore, the reference datum is the earth's horizontal plane rather than the aircraft longitudinal plane resulting in errors during banked turns.